The demand for increased bandwidth in fiberoptic telecommunications has driven the development of sophisticated transmitter lasers usable for dense wavelength division multiplexing (DWDM) systems wherein multiple separate data streams propagate concurrently in a single optical fiber. Each data stream is created by the modulated output of a semiconductor laser at a specific channel frequency or wavelength, and the multiple modulated outputs are combined onto the single fiber. The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz, which allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Greater bandwidth requirements will likely result in smaller channel separation in the future.
Telecom DWDM systems have largely been based on distributed feedback (DFB) lasers. DFB lasers are stabilized by a wavelength selective grating that is predetermined at an early step of manufacture. Unfortunately, statistical variation associated with the manufacture of individual DFB lasers results in a distribution of (wavelength) channel centers. Hence, to meet the demands for operation on the fixed grid of telecom wavelengths (the ITU grid), DFBs have been augmented by external reference etalons and require feedback control loops. Variations in DFB operating temperature permit a range of operating wavelengths enabling servo control; however, conflicting demands for high optical power, long lifetime, and low electrical power dissipation have prevented use in applications that require more than a single channel or a small number of adjacent channels.
Continuously tunable external cavity lasers have been developed to overcome the limitations of individual DFB devices. Tunable external cavity lasers, in order to provide effective side mode suppression and wavelength stability, require very stringent manufacturing tolerances. In order to meet these tolerances, expensive custom-made components are typically required for the external cavity lasers. Tuning has relied on the use of stepper motors to mechanically components, which reduces form factor, introduces vibration and shock sensitivity, reduces useful lifetime due to motor component wear, and increases the overall size and complexity of the external cavity lasers.
There is accordingly a need for an external cavity laser that is compact in size and has a small form factor, that is of simple, inexpensive construction, that provides for effective side mode suppression and wavelength stability during operation, that has reduced cavity loss and increased output power, and which has loose machining tolerances. The present invention satisfies these needs, as well as others, and overcomes the deficiencies found in the background art.
The invention provides external cavity lasers apparatus and methods that allow fast tuning, high wavelength stability, low cavity losses, and form factors that are comparable to solid state, fixed wavelength lasers. The apparatus of the invention comprises a gain medium emitting a light beam, a tunable element positioned in the light beam and configured feed back light of a selected wavelength to the gain medium, and a microelectromechanical systems (MEMS) actuator element operatively coupled to the tunable element. The MEMS actuator element may be configured to actuate the tunable element according to a first degree of freedom of movement to select the wavelength of the feedback to the gain medium, and to actuate the tunable element according to a second degree of freedom of movement to provide phase control of the feedback. The MEMS actuator element and tunable element may additionally be configured such that actuation of the tunable element with respect to a third degree of freedom of movement provides a selectable level of attenuation of the feedback to the gain medium.
The tunable element and MEMS actuator are configured to provide orthogonalized wavelength selection control and phase control of the feedback from the tunable element to the gain medium according to independent orthogonalized positional adjustments to the tunable element. In other words, wavelength tuning is uncoupled or decoupled from tuning of the external cavity tuning, such that the tuning mechanisms for wavelength selection and external cavity length adjustment operate independently or orthogonally with respect to each other. The adjustment of the wavelength passband thus has minimal effect on the effective cavity length, and adjusting the effective cavity length has minimal effect on the passband of the tunable element.
In certain embodiments, the tunable element may comprise a movable grating that is positioned in the light beam and configured to selectively feed back light to the gain medium according to positioning of the grating by the MEMS actuator. The grating is rotatable about a first axis to provide wavelength selection of the light fed back to the gain medium, and is translatable along a second axis to provide phase control of the light fed back to the gain medium. The rotational adjustment about the first axis to control wavelength selection is orthogonalized with respect to the translational adjustment along the second axis to control external cavity length, such that adjusting the grating for wavelength selection does not effect, or minimally effects, phase adjustment. Similarly, translatational adjustment of the grating along the second axis to provide phase control does not effect, or minimally effects, wavelength selection. The grating may additionally be rotatable about a third axis to provide attenuation control to the light fed back to the gain medium. The first axis may be parallel or substantially parallel to the grating face of the movable grating, and the second axis may be perpendicular or substantially perpendicular to the grating face and first axis. The third axis may be substantially parallel to the grating face, and substantially perpendicular to the first and second axes.
In some embodiments, the movable grating is a reflective grating and, together with a reflective facet of the gain medium, defines an external laser cavity. The grating may be etched or otherwise formed onto a MEMS mirror surface. The grating may be positioned with respect to the reflective facet of the gain medium such that external laser cavity is dimensioned to suppress lasing modes at wavelengths other than a selected wavelength. Specifically, the grating and gain medium may be positioned such that the external laser cavity is of sufficiently short length that the external cavity axial modes are spaced sufficiently far apart such that unwanted mode hopping from a selected wavelength to an external cavity mode will not occur during laser operation. The apparatus may, in certain embodiments, also comprise a mode filtering element positioned in the optical path, which may be in the form of an etalon configured to define a plurality of transmission peaks corresponding to selectable feedback wavelengths.
In other embodiments, the tunable element may comprise a movable mirror together with a stationary grating, with the movable mirror operatively coupled to the MEMS actuator element. The mirror is rotatable about a first rotational axis to control feedback wavelength and translatable along a second axis to control feedback phase. In certain embodiments, the mirror may additionally be rotatable about a third axis to control level of feedback attenuation.
The methods of the invention comprise emitting a light beam by a gain medium, positioning a tunable element in the light beam, coupling the tunable element to a microelectromechanical (MEMS) actuator, feeding back light to the gain medium by the tunable element, and positionally adjusting the tunable element with respect to a first degree of freedom of movement, via the MEMS actuator, to select wavelength of the light fed back to the gain medium. The methods may additionally comprise positionally adjusting the tunable element with respect to a second degree of freedom of motion to adjust phase of the light fed back to the gain medium. The positional adjusting with respect to the first and second degrees of freedom may be carried out orthogonally, such that positional adjustment of the tunable element to adjust wavelength does not affect phase adjustment provided by the tunable element. The methods may further comprise positionally adjusting the tunable element with respect to a third degree of freedom of movement to control attenuation of the light fed back to the gain medium.
The positioning of the tunable element in the light beam may in certain embodiments comprise positioning a reflective grating in the light beam. The grating may be etched, engraved or embossed or otherwise formed, using photolithographic or other technique, onto the surface of a MEMS-movable mirror that is coupled to a MEMS actuator. Positionally adjusting the grating to select wavelength of the feedback light may comprise rotatably actuating the grating with respect to a first axis that is parallel or substantially parallel to the grating face. Positionally adjusting the grating to select or adjust the phase of the feedback light may comprise translating the grating with respect to a second axis that is perpendicular or substantially perpendicular to a grating face thereof. The first and second axes are configured such that wavelength selection and phase selection are orthogonalized. Positionally adjusting the grating to control attenuation of or otherwise control the optical power of the feedback light may additionally comprise rotatably actuating the grating with respect to a third axis that is substantially perpendicular to the first and second axes.
In certain embodiments, the positioning of the grating in the light beam may comprise positioning the grating such that the grating and an reflective facet of the gain medium define an external laser cavity that is dimensioned to suppress lasing modes at wavelengths other than a selected wavelength. The grating and gain medium may be positioned such that the external laser cavity is of sufficiently short length that the external cavity axial modes are spaced sufficiently far apart such that unwanted mode hopping from a selected wavelength to an external cavity mode will not occur during laser operation. In some embodiments, the method may comprise positioning a mode filtering element in the light beam, and suppressing feedback at unselected wavelengths with the mode filtering element.
The apparatus and methods of the invention provide external cavity lasers that can be manufactured and assembled with relaxed tolerances and inexpensive components than is presently possible. The use of a MEMS actuator for positioning of a tunable element as provided by the invention allows shorter external cavity dimensions and smaller package sizes than have previously been achieved. In certain embodiments, the external cavity may be of sufficiently small dimension that effective suppression of unwanted wavelengths is achieved without the use of an intracavity filter or mode suppression element. The multiple degrees of freedom of movement of the tunable element allow wavelength selection, phase control and output power control during laser operation by appropriate actuation of the tunable wavelength reflection element. Adjustment to provide wavelength selection and phase control may be carried out independently or orthogonally, such that adjustments to a tunable element to provide wavelength selection minimally affect external cavity length or phase control adjustment. The short external cavity length and use of MEMS actuation also allows dynamic provisioning and rapid tuning or adjustment of output wavelength, feedback phase, and output power during laser operation. These and other objects and advantages of the invention will be apparent from the detailed description below.